This post will briefly explain AC and DC and how we change one form of power to another. Later, It discusses the advantages and disadvantages of both methods of charging a solar battery. It will also point out a unique problem that micro-inverters such as Enphase micro-inverters face by locking you into one style of charging. 

AC AND DC – THE BASICS

DC POWER

DC stands for “direct current”.  Direct current flows in one direction; electrons move from negative to positive. It’s the type of electricity that comes from your solar panels, but it is also the electricity that comes from any kind of battery. From car batteries to watch batteries, AA batteries to Solar Batteries; all batteries are really DC batteries.

AC POWER

AC stands for alternating current. In Australia, that current alternates or changes direction 50 times a second, or at 50Hz. Alternating current is the type of electricity that comes from the poles and wires on the streets, into your home, and out of your power points.

CHANGING FROM AC TO DC

While your solar panels produce DC electricity, that DC is “inverted” (or converted) into AC power by the solar inverter. We then use this AC power for every appliance in our homes.

However, mobile phones, laptops,  modems, tv’s and many other appliances use DC power. So why can we plug these appliances into an AC powerpoint? These devices convert the AC power back to DC power – with an AC to DC rectifier. A rectifier does the reverse of an inverter.

AC-coupled battery example 1. Installation of A tesla Power Wall. Connection is made through the switchboard with not direct connection to an exsiting or new solar inverter

AC COUPLING A SOLAR BATTERY

It’s no different with an AC coupled solar battery like the Tesla Powerwall, the Enphase AC battery, or the SonnenBatterie. AC coupled batteries store DC energy; they are just all charged by being “coupled” or “plugged into” AC power.

You’ll still need either a standard string inverter such as a Fronius, or micro-inverters like Enphase. We connect your AC battery to work alongside – but independently of your inverter.

The AC coupled battery has a built-in rectifier/inverter that converts power from the AC power source to DC power in order to charge the battery.

When it is time to discharge the battery, the battery uses the inbuilt inverter to convert the battery’s DC energy back to AC so that it can be used again by the appliances in your home.

So an AC coupled solar battery uses three stages of conversion:

1. DC (from the solar panels) to AC (at the solar inverter)
2. AC (from the solar inverter) to DC (to charge the battery)
3. DC (from the battery) to AC (to be used in your home).

DC COUPLING A SOLAR BATTERY

In contrast, DC coupling a solar battery only requires power conversion once rather than three times. It uses DC power from the solar panels to charge the battery. It then converts the DC power from the battery back to AC power to be used by the home.

Both of these steps normally happen by what is commonly called a “hybrid inverter”. A hybrid inverter is simply a battery charger, and a solar inverter put together in one box.

The battery charger must communicate with the battery to charge and discharge the solar battery at the right rate. What happens when it doesn’t?

DISADVANTAGES OF DC COUPLING A SOLAR BATTERY

You might be thinking DC coupling is a better solution. Why would you get an AC Battery that goes: DC-AC-DC-AC. 

Several years ago, it became apparent to the industry that the future of the solar boom would include household solar batteries. It wasn’t long before every inverter manufacturer had their own version of a Hybrid inverter, and every lithium battery manufacturer had their own version of a lithium solar battery.

The lithium battery manufacturers and inverter manufacturers each chose several partners that they wanted to work with. All they had to do was get the communications between the battery and the hybrid inverter right. The lithium battery just needed talk with the inverter to ensure it was charged and discharged at the right rate. How hard can it be?

But it’s a long way to the top if you want to rock and roll. Communications between Brand A and Brand B turned out harder than you might think. As technology rapidly developed, software upgrades were continually done on both the inverter and the charger.  When the installation failed, or lithium battery didn’t perform as guaranteed, the finger-pointing game between the battery and the inverter began. The installer and the customer were both hung out to dry. And we’re not just talking about pop-up brands. These warranty issues are endemic even amongst the well-known brands.

AC coupled batteries, however, don’t have the same issue because the inverter/charger is built into the battery. However, AC coupling does have another significant issue in many homes around Australia. It’s to do with regulations.

DC-coupled battery example 1. Install an 10kW Fronius Gen24 Symo Plus Coupled with 12kW of solar panels, in this case REC Alpha.

The Fronius Gen24 is DC coupled to BYD Home battery, 2 stacks of 11KWh. 

DISADVANTAGES OF AC COUPLING A SOLAR BATTERY

In the NT, we are limited to a maximum of 10kW of inverters on each phase. The problem is, the 10 kW limit includes the sum of all the inverters and the AC coupled inverter/battery charger. On single-phase, if you want to have a large 8.2kW Fronius solar inverter with 10kW of panels, you are left with a piddly 1.8kW AC coupled battery inverter. In reality, if you need a larger solar system with a larger than 5kW solar inverter, you’ll need to use a DC-coupled battery …. if you can.

If however, you chose an Enphase microinverter system, you don’t have the option to DC Couple a battery, because the DC power is inverted to AC behind each solar panel. So if you choose Enphase, you can only ever install an AC coupled battery. Consider the following examples:

C coupled battery example 1. Install a healthy 10kW solar system with 8kW total capacity of micro-inverters on your roof.
The problem? You will only be allowed to install a (future) 4kWh Enphase battery with a 2kW inverter charger. A 4kWh battery is a bit of a token effort.

AC Coupled battery example 2. Install 6.6kW of solar panels on 5kW Fronius inverter. Later, install a 13.5 kWh Tesla battery with a 5kWh inverter charger.

The problem? It’s likely you won’t have enough excess solar to charge your battery.

ADVANTAGES OF DC COUPLING A SOLAR BATTERY

If you choose to install anything other than Micro-inverters, you leave your options open to either an AC coupled or DC coupled battery in the future. Under current NT regulations, DC coupling allows you to install a 10kW inverter, 15kW of panels, and any size battery that you like. This is because a DC-coupled system does not need an additional inverter/charger just to charge the battery. We just use a DC battery charger (the “hybrid” part of a hybrid inverter).

DC-coupled battery example 1. Installation of 10kVA Fronius Gen24 Primo Inverters with one 5KVA in verter connected to a BYD Home battery.

10-15kW of solar panels producing 60kWh per day.

 In 5 years when your ready for batteries , upgrade the firmware in the 10kW Fronius Gen24 to allow for batteires and add any quantitiy of BYD HVM battery. 

ADVANTAGES OF AC COUPLING A SOLAR BATTERY

In essence, an AC coupled solar battery is a battery with an inverter charger built-in. One obvious benefit of this is there can be no finger-pointing. If there are issues with either the Powerwall battery or the Powerwall inverter/charger  – it’s a Tesla issue.

The option of AC coupling also allows you to install an affordable string inverter now, and keep it for the ten year warranty period. If you choose to install a battery five years down the track, just wack in whatever battery comes on the market without being concerned about compatibility (assuming you are keeping to the local network’s regulations).

The other clear advantage of the battery and the inverter not being interdependent is if either the inverter or battery fails five years down the track, you can replace that product with an updated product without being concerned about compatibility.